I believe that it is an inherent problem with the E420. That was the first year that they went to Rack & Pinion with the V-8. I think that the shimmy was always there, but previously masked by the recirculating ball power steering found on the 400E/E420's.

If its any consolation I've spoken with many BMW techs that have also noticed the same thing with the BMW 5-Series. The 528i has a shimmy problem (rack & pinion), while the 540i does not (recirculating ball).